Allow builds on OpenBSD

[qth]

After making these changes, I was then able to build mapnik on OpenBSD.

Just in case anyone feels crazy and wants to try on OpenBSD for themselves ;-), this was the configure line that worked for me:

./configure ICU_LIBS=/usr/local/lib ICU_INCLUDES=/usr/local/include HB_LIBS=/usr/local/lib HB_INCLUDES=/usr/local/include PROJ_LIBS=/usr/local/lib PROJ_INCLUDES=/usr/local/include FREETYPE_LIBS=/usr/X11R6/lib FREETYPE_INCLUDES=/usr/X11R6/include/freetype2 CUSTOM_CFLAGS=-DACCEPT_USE_OF_DEPRECATED_PROJ_API_H=1 -I/usr/X11R6/include/freetype2 CUSTOM_CXXFLAGS=-DACCEPT_USE_OF_DEPRECATED_PROJ_API_H=1 -I/usr/X11R6/include/freetype2

[inferiorhumanorgans]

I've recently gone down the path of trying to build mapnik on FreeBSD. It seems to me like leveraging pkgconf/pkg-config would be useful for cross platform compatibility and minimally intrusive on Linux.

[hummeltech]

@inferiorhumanorgans, mapnik is available again in FreeBSD since last year.
https://github.com/freebsd/freebsd-ports/tree/36cc90cb698e00ffdb8f81a64d2d5f8cf34ead45/graphics/mapnik

I suspect it would be a similar build process for OpenBSD.